Thermoplastic polyesters, such as poly(ethylene terephthalate) [PET], poly(butylene terephthalate) [PBT], and poly(1,4-cyclohexylenedimethylene terephthalate) [PCT] are useful as injection molding compounds for a variety of applications. In general these materials are reinforced with glass fiber or mineral fillers to enhance properties. Applications include automotive parts, appliance parts, and electrical-electronics parts. The reinforced thermoplastic polyesters have useful combinations of strength, heat resistance and chemical resistance for these uses. In many cases it is required that the material also be flame-retarded, and thermoplastic polyesters can be successfully flame retarded through the use of a number of known flame-retarding additives.
Many times the thermoplastic polyester is processed into the final article of interest by injection molding. In the injection molding process, a quantity of material is melted but only a portion of it is injected into the mold. It is possible that material will remain in the melt in the molding machine for several minutes (perhaps fifteen minutes) before it is injected and cooled. For this reason, it is desirable for the polymer to have excellent stability so that it will maintain high molecular weight even after several minutes holding time at melt temperatures. Maintaining high molecular weight (or I.V.) is important in maintaining mechanical properties such as tensile, impact and flexural strengths.
This problem of melt stability is especially significant for PCT because of its higher melting temperature (290.degree. C. vs 250.degree. C. for PET and 225.degree. C. for PBT). The higher melting temperature means that the polymer must be processed at a higher temperature, which accelerates the degradation rate.
One method to maintain the molecular weight at melt temperatures is to compound reactive additives into the formulation. Useful known additives of this type are multifunctional epoxy compounds and oligomers or polymers produced from them. These additives help maintain molecular weight by reacting with polyester chain ends. Because the additives are multifunctional, however, they will also result in chain branching. Thus an initially linear thermoplastic polyester, after stabilization with the multifunctional additive and exposure to melt conditions, will be branched. This is an undesirable result because it leads to changing flow characteristics and irreproducible processing and mechanical properties.
Branching can be controlled by the use of phosphorous-based stabilizers in conjunction with the reactive epoxy compounds. In some cases, however, branching is controlled at the expense of melt stability; i.e. the effectiveness of the reactive epoxy-based compound is compromised. Unexpectedly, it has been found that a certain class of phosphorous stabilizers will allow branching to be controlled without unacceptable loss of melt stability.
The use of certain epoxy compounds in PCT is disclosed in European Patent Application 0 273 149. This application also discloses the use of a phosphate compound as a component of a formulation. Phosphates are not within the scope of the present invention. The use of phenoxy resin, an epoxy-based polymer, in PET and other thermoplastic polyesters is known by others.